1. Field of the Invention
The invention relates to a detection device for surgical objects within the human body. More specifically the invention relates to a detection device which responds to an alternating electromagnetic field.
2. Description of the Prior Art
Despite precautions, surgeons still occasionally leave surgical objects such as sponges and, less frequently, small surgical tools in their patients after an operation. Areas which are badly injured tend to have a great amount of blood which may cover the surgical objects making the objects hard to locate. Also, objects may find their way under an organ. This is most likely to occur in surgical areas such as the abdomen which are large and have many organs.
The prior art discloses use of X-ray opaque material positioned on the surgical devices in order that after the surgery is completed and the wound is closed an X-ray can be taken to insure no surgical objects were left within the patient. Although this detection method is effective, it is cumbersome. Most operating rooms do not have X-ray machines. Hence, the patient must be taken to another room. There the patient often must be moved from his gurney to an X-ray table for X-rays to be taken. If a surgical object is detected after an X-ray has been taken, the patient must be returned to the operating room. Then, the cavity or incision must be reopened to remove the surgical object and then reclosed. This second surgery can cause a great deal of trauma to the patient preventing optimum healing. Examples of surgical sponges which are marked by radio opaque material are disclosed in U.S. Pat. No. 2,190,432 to Lewison, U.S. Pat. No. 2,698,270 to Mesek, U.S. Pat. No. 4,185,626 to Jones et at., and U.S. Pat. No. 4,205,680 to Marshall.
A manual counting of the sponges after the surgery is completed is also used to prevent surgical objects from being left in body cavities. This is not a foolproof method. Fatigue, poor handwriting, and misreading of numbers will occur during operations lasting 4 to 12 hours when dealing with badly damaged patients. Consequently, miscounts occur as a result of human error.
Greenberg in U.S. Pat. No. 3,587,583 attempts to overcome the problems of leaving surgical objects within the body. He proposes to mark the surgical object with a permanently magnetized material. A surgeon performs an operation in the normal manner. Before closing the incision the surgeon probes for the presence of a surgical object with a magnetic field detector means which generates an electric signal which is modified in the presence of a magnetic field. If the marked object is present, the magnetic field of the magnetic marker is sensed by the magnetic field detector means which modifies the electric signal. Yet, an operating room has many types of equipment which generate permanent magnetic fields. The presence of those fields can activate the magnetic field detector means giving false detection. Because of its unreliability in an operating room, Greenberg's device is not a practical solution to the problem.
In U.S. Pat. No. 5,057,095, Fabian proposes to mark surgical instruments with a marker adapted to produce identifying signal characteristics when exposed to an alternating magnetic field. He discloses three types of resonant markers that are able to resonate at a certain preselected frequency. The first marker is a magnetomechanical device comprised of a permanent magnet overlaying a magnetostrictive metal strip in a plastic housing. The magnetostrictive strip vibrates when the marker is exposed to an alternating electomagnetic field and its resonance is detected when the frequency of the applied field reaches a predetermined value. However, such devices are very sensitive to pressure and stress which will inhibit them. Since a body cavity is under some pressure and the marker may be stressed during surgery, this type of marker is not reliable for use as a marker for surgical objects. The second proposed type is an electromechanical circuit comprised of an air coil, with or without a ferrite core and a resonant structure such as a piezoelectric crystal. As the first type, this type of marker can be adversely affected by pressure and stress because its principle of detection relies on a mechanical resonance, therefore a piezoelectric crystal type marker is also unsatisfactory. The third type of marker proposed by Fabian is an electromagnetic LCR circuit. This type of marker can be either built out of discreet components or made of a flexible printed circuit. In the former case, this unit is expensive to build and bulky and it is impractical for surgical sponges. In the later case, due to its high electrical resonance frequency this type of marker can be adversely affected by the presence of metal objects and conductive media. Because the human body is conductive, it is also impractical for surgical sponges. Consequently, none of the markers proposed by Fabian, nor the Greenberg marker, has been available on the market.
In U.S. Pat. No. 5,045,071 McCormick teaches about the use of magnetic materials for accurately locating the position of a catheter which has been inserted into a blood vessel. At column 9, lines 12-16, the patent cross references U.S. Pat. Nos. 4,416,289; 4,431,005 and 4,445,501 for an explanation of the general method of detection. At column 5, lines 41-52, the '005 patent explains that a distortion of the magnetic field indicates the presence of the catheter. Thus, the McCormick patent teaches that merely a change in the magnetic field is a sufficient indicator of the position of the marked object. However, McCormick's measurements can be affected by the presence of other nearby magnetic and conductive materials. Hence, McCormick's technique can and likely will provide "false positives" as to the presence or the position of the marked object.
Thus, there is a need for a marking method and apparatus for detecting surgical objects within the human body utilizing a material that can be readily identified before the patient leaves the operating suite.
3. Techniques for Detecting Electromagnetic Material
There are different ways of providing and detecting what we can call generically an "electromagnetic marker." The cited prior art references all use materials which respond to an electromagnetic field. In order for a material to respond to an electromagnetic field and therefore to create "detectable changes" of the electromagnetic field, a material has to have at least one of the physical properties of electrical conductivity, moderate to high magnetic permeability, and magnetrostriction (in general associated with moderate magnetic permeability). Moderate magnetic permeability is defined as a permeability comprised of between 5,000 and 20,000 and high magnetic permeability as a permeability above 20,000. In each case, the response to the electromagnetic field and, therefore, the creation of "detectable changes" of the electromagnetic field are heavily dependent upon the geometry and size of the marker. In addition, the response to the electromagnetic field depends upon the intensity and frequency of the electromagnetic field.
In general a magnetic material subject to an electromagnetic field of known and fixed frequency f.sub.o responds to the applied electromagnetic field by creating "changes" of the intensity of the applied field and by creating harmonics of the frequency f.sub.o. If the material is electrically conductive, it responds by creating not only "changes" of the intensity of the applied field but also "changes" of the phase of the field. In addition, if the material is magnetostrictive, the electromagnetic field creates strains or stress in the material, and the material responds to it by creating a frequency-dependent "change" of the intensity and of the phase of the applied field. Therefore, there are three methods of detection. First, one can simply look for a change of intensity and/or phase in an applied magnetic field, a method which can only be used for detection of the position of an object at a distance comparable to the size of the object. McCormick uses this method. Second, one could look for the frequency of the applied field to reach a predetermined value that is the electromechnical resonance frequency of the marker. Fabian discloses a magnetomechanical device which uses this technique. Finally one could look for particular harmonics generated by a material in the presence of an applied magnetic field. This method has never been used in a medical environment. Indeed, the teaching of Heltemes in U.S. Pat. No. 4,857,891, indicates that the art has generally failed to recognize that "open-strip" markers made of selected nonmagnetostrictive materials which generate specific harmonic frequencies upon application of a unidirectional electromagnetic field can be used to identify the presence of particular articles.
Heltemes discloses a Magnetic Marker for Electronic Article Surveillance Systems having multiple filaments randomly dispersed in a sheet-like substrate so as to be substantially parallel to the plane thereof. "The filaments are selected of low coercive force, high permeability material, and the random orientation results in certain filaments intersecting with them being magnetically coupled to other filaments to thereby collect and concentrate lines of flux associated with an applied field of an EAS system into filaments parallel to the field."
To take advantage of a high magnetic permeability material a marker has to be elongated (fiber, long strip, with an aspect ratio length/square root or cross sectional area of a least 200). Heltemes complies only partially to this requirement, but randomly distributes the fibers. In this respect Heltemes defeats this purpose because the applied electromagnetic field has to be parallel to the magnetic fibers to generate a high enough level of high harmonics to be recognized as marker specific. Moreover, Heltemes' marker is not very well suited for generating high harmonics. Consequently, Heltemes like others in the prior art, failed to recognize that markers could be created for detection of surgical objects which generate specific, detectable, selected harmonic frequencies.